MAX6675ISA [MAXIM]
Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0∑C to +1024∑C); 冷端补偿的K - Thermocoupleto数字转换器（ 0ΣC至+ 1024ΣC ）
| 型号： | MAX6675ISA |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0∑C to +1024∑C) |
| 文件： | 总8页 (文件大小：131K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-2235; Rev 1; 3/02
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
General Description
Features
The MAX6675 performs cold-junction compensation
and digitizes the signal from a type-K thermocouple.
The data is output in a 12-bit resolution, SPI™-compati-
ble, read-only format.
o Direct Digital Conversion of Type -K
Thermocouple Output
o Cold-Junction Compensation
o Simple SPI-Compatible Serial Interface
o 12-Bit, 0.25°C Resolution
This converter resolves temperatures to 0.25°C, allows
readings as high as +1024°C, and exhibits thermocou-
ple accuracy of 8LSBs for temperatures ranging from
0°C to +700°C.
o Open Thermocouple Detection
The MAX6675 is available in a small, 8-pin SO package.
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX6675ISA
-20°C to +85°C
8 SO
Applications
Pin Configuration
Industrial
Appliances
HVAC
TOP VIEW
GND
T-
1
2
3
4
8
7
6
5
N.C.
SO
Automotive
MAX6675
T+
CS
V
SCK
CC
SO
SPI is a trademark of Motorola, Inc.
Typical Application Circuit
Vcc
0.1 F
MAX6675
MICROCONTROLLER
68HC11A8
GND
MISO
SO
SCK
CS
T+
T-
SCK
SSB
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V
to GND) ................................ -0.3V to +6V
Storage Temperature Range ............................-65°C to +150°C
Junction Temperature .................................................... +150°C
SO Package
CC
SO, SCK, CS, T-, T+ to GND .......................-0.3V to V
SO Current ........................................................................ 50mA
+ 0.3V
CC
ESD Protection (Human Body Model) ........................... 2000V
Vapor Phase (60s) . .....................................................+215°C
Infrared (15s) ..............................................................+220°C
Lead Temperature (soldering, 10s) ............................... +300°C
Continuous Power Dissipation (T = +70°C)
A
8-Pin SO (derate 5.88mW/°C above +70°C) .............. 471mW
Operating Temperature Range ..........................-20°C to +85°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
= +3.0V to +5.5V, T = -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1)
A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
-5
TYP
MAX
+5
UNITS
V
V
V
V
= +3.3V
= +5V
CC
CC
CC
CC
T
= +700°C,
THERMOCOUPLE
T = +25°C (Note 2)
A
-6
+6
= +3.3V
= +5V
-8
+8
T
= 0°C to
THERMOCOUPLE
+700°C, T = +25°C (Note 2)
A
-9
+9
Temperature Error
LSB
T
= +700 C
V
V
= +3.3V
= +5V
-17
-19
+17
+19
THERMOCOUPLE
CC
CC
to +1000 C, T = +25 C
A
(Note 2)
Thermocouple Conversion
Constant
10.25
µV/LSB
V
V
= +3.3V
= +5V
-3.0
-3.0
+3.0
+3.0
CC
CC
Cold-Junction
Compensation Error
T = -20°C to +85°C
A
(Note 2)
°C
°C
k
Resolution
0.25
60
Thermocouple Input
Impedance
Supply Voltage
V
3.0
1
5.5
1.5
2.5
V
mA
V
CC
Supply Current
I
0.7
2
CC
Power-On Reset Threshold
Power-On Reset Hysteresis
Conversion Time
V
rising
CC
50
mV
s
(Note 2)
0.17
0.22
SERIAL INTERFACE
0.3 x
Input Low Voltage
Input High Voltage
V
V
V
IL
V
CC
0.7 x
V
IH
V
CC
Input Leakage Current
Input Capacitance
I
V
= GND or V
CC
5
µA
pF
LEAK
IN
C
5
IN
2
_______________________________________________________________________________________
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
ELECTRICAL CHARACTERISTICS (continued)
(V
= +3.0V to +5.5V, T = -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1)
A
CC
PARAMETER
SYMBOL
CONDITIONS
= 1.6mA
SOURCE
MIN
TYP
MAX
UNITS
V
-
CC
Output High Voltage
V
I
I
V
V
OH
0.4
Output Low Voltage
TIMING
V
= 1.6mA
SINK
0.4
4.3
OL
Serial Clock Frequency
SCK Pulse High Width
SCK Pulse Low Width
CSB Fall to SCK Rise
CSB Fall to Output Enable
CSB Rise to Output Disable
f
MHz
ns
SCL
t
100
100
100
CH
t
ns
CL
t
C = 10pF
L
ns
CSS
t
C = 10pF
L
100
100
ns
DV
t
C = 10pF
L
ns
TR
SCK Fall to Output Data
Valid
t
C = 10pF
L
100
ns
DO
Note 1: All specifications are 100% tested at T = +25°C. Specification limits over temperature (T = T
to T
) are guaranteed
MAX
A
by design and characterization, not production tested.
Note 2: Guaranteed by design. Not production tested.
A
MIN
Typical Operating Characteristics
(V
= +3.3V, T = +25°C, unless otherwise noted.)
CC
A
OUTPUT CODE ERROR
vs. AMBIENT TEMPERATURE
OUTPUT CODE ERROR
vs. VOLTAGE DIFFERENTIAL
10
10
8
6
4
2
5
0
-5
0
0
15
30
45
60
75
90
-10
0
10
20
30
40
50
TEMPERATURE ( C)
VOLTAGE DIFFERENTIAL (mV)
_______________________________________________________________________________________
3
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Where:
Pin Description
V
is the thermocouple output voltage (µV).
OUT
PIN
NAME
FUNCTION
T
is the temperature of the remote thermocouple junc-
R
tion (°C).
1
GND
Ground
T
is the ambient temperature (°C).
AMB
Alumel Lead of Type-K Thermocouple.
Should be connected to ground
externally.
2
T-
Cold-Junction Compensation
The function of the thermocouple is to sense a differ-
ence in temperature between two ends of the thermo-
couple wires. The thermocouple‘s hot junction can be
read from 0°C to +1023.75°C. The cold end (ambient
temperature of the board on which the MAX6675 is
mounted) can only range from -20°C to +85°C. While
the temperature at the cold end fluctuates, the
MAX6675 continues to accurately sense the tempera-
ture difference at the opposite end.
3
4
T+
Chromel Lead of Type-K Thermocouple
Positive Supply. Bypass with a 0.1µF
capacitor to GND.
V
CC
5
6
SCK
Serial Clock Input
Chip Select. Set CS low to enable the
serial interface.
CS
7
8
SO
Serial Data Output
No Connection
The MAX6675 senses and corrects for the changes in
the ambient temperature with cold-junction compensa-
tion. The device converts the ambient temperature
reading into a voltage using a temperature-sensing
diode. To make the actual thermocouple temperature
measurement, the MAX6675 measures the voltage from
the thermocouple’s output and from the sensing diode.
The device’s internal circuitry passes the diode’s volt-
age (sensing ambient temperature) and thermocouple
voltage (sensing remote temperature minus ambient
temperature) to the conversion function stored in the
ADC to calculate the thermocouple’s hot-junction tem-
perature.
N.C.
Detailed Description
The MAX6675 is a sophisticated thermocouple-to-digi-
tal converter with a built-in 12-bit analog-to-digital con-
verter (ADC). The MAX6675 also contains cold-junction
compensation sensing and correction, a digital con-
troller, an SPI-compatible interface, and associated
control logic.
The MAX6675 is designed to work in conjunction with an
external microcontroller (µC) or other intelligence in ther-
mostatic, process-control, or monitoring applications.
Optimal performance from the MAX6675 is achieved
when the thermocouple cold junction and the MAX6675
are at the same temperature. Avoid placing heat-gener-
ating devices or components near the MAX6675
because this may produce cold-junction-related errors.
Temperature Conversion
The MAX6675 includes signal-conditioning hardware to
convert the thermocouple’s signal into a voltage compat-
ible with the input channels of the ADC. The T+ and T-
inputs connect to internal circuitry that reduces the intro-
duction of noise errors from the thermocouple wires.
Digitization
The ADC adds the cold-junction diode measurement
with the amplified thermocouple voltage and reads out
the 12-bit result onto the SO pin. A sequence of all
zeros means the thermocouple reading is 0°C. A
sequence of all ones means the thermocouple reading
is +1023.75°C.
Before converting the thermoelectric voltages into
equivalent temperature values, it is necessary to com-
pensate for the difference between the thermocouple
cold-junction side (MAX6675 ambient temperature) and
a 0°C virtual reference. For a type-K thermocouple, the
voltage changes by 41µV/°C, which approximates the
thermocouple characteristic with the following linear
equation:
✕
V
= (41µV / °C) (T - T
)
AMB
OUT
R
4
_______________________________________________________________________________________
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
mounting technique, and the effects of airflow. Use a
Applications Information
large ground plane to improve the temperature mea-
Serial Interface
The Typical Application Circuit shows the MAX6675
interfaced with a microcontroller. In this example, the
MAX6675 processes the reading from the thermocou-
ple and transmits the data through a serial interface.
Force CS low and apply a clock signal at SCK to read
the results at SO. Forcing CS low immediately stops
any conversion process. Initiate a new conversion
process by forcing CS high.
surement accuracy of the MAX6675.
The accuracy of a thermocouple system can also be
improved by following these precautions:
Use the largest wire possible that does not shunt
heat away from the measurement area.
If small wire is required, use it only in the region of
the measurement and use extension wire for the
region with no temperature gradient.
Force CS low to output the first bit on the SO pin. A
complete serial interface read requires 16 clock cycles.
Read the 16 output bits on the falling edge of the clock.
The first bit, D15, is a dummy sign bit and is always
zero. Bits D14–D3 contain the converted temperature in
the order of MSB to LSB. Bit D2 is normally low and
goes high when the thermocouple input is open. D1 is
low to provide a device ID for the MAX6675 and bit D0
is three-state.
Avoid mechanical stress and vibration, which could
strain the wires.
When using long thermocouple wires, use a twisted-
pair extension wire.
Avoid steep temperature gradients.
Try to use the thermocouple wire well within its tem-
perature rating.
Use the proper sheathing material in hostile environ-
ments to protect the thermocouple wire.
Figure 1a is the serial interface protocol and Figure 1b
shows the serial interface timing. Figure 2 is the SO out-
put.
Use extension wire only at low temperatures and
only in regions of small gradients.
Open Thermocouple
Bit D2 is normally low and goes high if the thermocou-
ple input is open. In order to allow the operation of the
open thermocouple detector, T- must be grounded.
Make the ground connection as close to the GND pin
as possible.
Keep an event log and a continuous record of ther-
mocouple resistance.
Reducing Effects of Pick-Up Noise
The input amplifier (A1) is a low-noise amplifier
designed to enable high-precision input sensing. Keep
the thermocouple and connecting wires away from
electrical noise sources.
Noise Considerations
The accuracy of the MAX6675 is susceptible to power-
supply coupled noise. The effects of power-supply
noise can be minimized by placing a 0.1µF ceramic
bypass capacitor close to the supply pin of the device.
Chip Information
TRANSISTOR COUNT: 6720
PROCESS: BiCMOS
Thermal Considerations
Self-heating degrades the temperature measurement
accuracy of the MAX6675 in some applications. The
magnitude of the temperature errors depends on the
thermal conductivity of the MAX6675 package, the
_______________________________________________________________________________________
5
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
CS
SCK
D0
SO
D15
D14
D13
D12
D7
D6
D4
D2
D11
D10
D9
D8
D5
D3
D1
Figure 1a. Serial Interface Protocol
t
CSS
CS
t
t
CL
CH
SCK
SO
t
DV
t
DO
t
TR
D15
D3
D2
D1
D0
Figure 1b. Serial Interface Timing
DUMMY
BIT
12-BIT
TEMPERATURE READING
THERMOCOUPLE DEVICE
STATE
SIGN BIT
INPUT
ID
Bit
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Three-
state
0
MSB
LSB
0
Figure 2. SO Output
6
_______________________________________________________________________________________
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Block Diagram
V
CC
0.1 F
4
DIGITAL
CONTROLLER
COLD-JUNCTION
COMPENSATION
DIODE
S5
5
SCK
300k
ADC
7
6
30k
30k
S3
T+
T-
SO
CS
1M
3
2
S2
A2
A1
S4
S1
20pF
MAX6675
300k
REFERENCE
VOLTAGE
1
GND
_______________________________________________________________________________________
7
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Package Information
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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